1. Field of the Invention
The invention relates to electrical melting techniques for glass and, more specifically, to those wherein the conductivity of the molten glass is used to develop the energy necessary to melt the raw materials.
2. Discussion of the Background
For a long time, installations for high volume glass production have been equipped with melting furnaces using fossil fuel and gas in particular. This is particularly the case for high capacity, continuous production installations which produce, for example, flat glass or glass for bottles. When electrical energy was used in those large furnaces, it was basically used locally to maintain the temperature of the glass in the less hot zones or outside the furnace along the glass' path to its transformation area or, even, to develop certain convection movements which supposedly assist the homogenization, the refining or the movement of the molten material.
Electrical melting per se first appeared on small units for which high flexibility in use seemed necessary. Fluctuations in energy costs and the progressive mastery of certain technological problems have led more recently to the development of high production units in which the entire melting process, except for the start-up, takes place using electrical energy. This development requires solving extremely delicate technological problems.
Consequently and particularly in order to avoid the question of oxidation of the electrodes at the surface of the melting bath, it has been proposed to immerse them completely. This is the solution adopted in the French patent No. FR-A-2 552 073. In this document, the electrodes are arranged vertically in the bath from the floor of the furnace. In other embodiments, there are also electrodes passing through the lateral walls of the furnace.
Independent of the advantages it offers with respect to corrosion problems, immersion of the electrodes also provides a convenient and very uniform supply of raw materials to the surface of the bath. The constitution of a relatively thick layer of composition to be melted, floating on the molten bath, is in effect useful for several reasons. On contact with the molten bath, it forms a permanent reserve of material necessary for continuous operation. It also protects the molten bath from high caloric depletion through convection on contact with the atmosphere and, in particular, through radiation.
While the furnaces of the type described in the above-identified document have very considerable industrial applications, they do not necessarily respond in the best manner to all requirements encountered in practice. By way of example, it is desirable, in certain cases and with an obvious aim of limiting investment costs, to transform the installations using burners by retaining as many as possible of the existing elements and in particular the refractory materials constituting the tank. Such a transformation is not possible when electrodes are to be implanted in the floor or the lateral walls of the furnace.
The furnaces whose electrodes are immersed offer limited possibilities for regulating the electrodes. While they provide completely satisfactory results for a certain regimen, they adapt less well to frequent and/or substantial modifications to this system of operation.
In addition, even if the technology of immersed electrodes is now well developed and a life span for the electrodes which is comparable to that of the refractories can be foreseen, the possibility of premature deterioration of one or several electrodes, which would compromise good operation, cannot be completely ruled out.